1. Field of the Invention
The invention relates to a coil for an electromagnetic relay having a coil former which forms a coil tube, whose basic shape is cylindrical, as a support for a winding stack as well as two flanges as axial boundaries of the winding space in each case.
2. Description of the Related Art
In the case of electromagnetic relays, a round magnetic soft-iron core is frequently used inside the excitation winding to guide the flux, for which reason the coil former, which is composed of insulating material and supports the winding, likewise has a coil tube of a cylindrical shape. The winding, which is composed of round copper wires which are thermoplastically sheathed, is fitted under tension into the winding space on this coil tube between both flanges so that the winding stack is initially seated firmly and such that it cannot move on the coil tube. The winding ends are connected, for example, to connecting wires in the coil flanges, strain relief normally being provided in order that the winding ends, which are subject to tension, do not tear in the event of temperature-dependent changes or changes produced by mechanical stress.
However, as a result of temperature and time influences, the winding shrinks because of the wire insulation shrinking, and this means that the external diameter and the length of the winding become smaller while the internal diameter becomes larger. In consequence, the winding stack becomes loose on the coil tube and can move in the axial and radial directions. As a result of additional forces acting from the outside, such as vibration, shock stress and temperature change, there is a risk of an interruption in the winding wire ends between the winding and the connecting ends which are anchored, for example, firmly in a coil former flange. Such vibration stresses occur particularly when a relay is installed in a moving apparatus, for example in a motor vehicle.